Multi-frequency ground monitor current sensing without a dc component

ABSTRACT

An apparatus includes a signal generation module that injects a signal in a pilot conductor with one or more AC components. Pilot conductor current is injected in a grounding conductor of power cables connecting a source to a load. A current monitor module monitors affected current in the pilot conductor. A DC detection module determines a DC current present in the current. A DC minimum threshold module determines if the DC current is below a DC current threshold. An AC detection module determines an AC current of each frequency of the AC components. An AC threshold module determines if any of the AC currents is below an AC threshold. A trip module opens a contact after determining that the DC current is below the DC current minimum threshold and/or determining that at least one of the AC currents is below an AC threshold. The contact disconnects the source.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent application Ser. No. 14/473,568 entitled “MULTI-FREQUENCY GROUND MONITOR CURRENT SENSING” and filed on Aug. 29, 2014 for Dale Curtis, which claims the benefit of U.S. Provisional Patent Application No. 61/871,448 entitled “MULTI-FREQUENCY GROUND MONITOR CURRENT SENSING” and filed on Aug. 29, 2013 for Dale Curtis, which are incorporated herein by reference. U.S. patent application Ser. No. 14/823,959 entitled “MULTI-FREQUENCY GROUND MONITOR CURRENT SENSING” and filed on Aug. 11, 2015 for Dale Curtis, attorney docket no. 3129.2.3C, and U.S. patent application Ser. No. 14/824,006 entitled “FREQUENCY HOPPING GROUND MONITOR CURRENT SENSING” and filed on Aug. 11, 2015 for Dale Curtis, attorney docket no. 3129.2.3D, are incorporated herein by reference for all purposes.

FIELD

This invention relates to monitoring ground current and more particularly relates to using a multi-frequency generator when monitoring ground current and using frequency hopping for the frequencies.

BACKGROUND

Mining is a very special environment that is by its very nature hazardous. Mine shafts are very limited physically, often include wet conditions, and can have explosive gases and dust. The mining industry has a long history of accidents and fatalities. As a result, governmental regulations as well as company policies are geared toward making mining safer for those that enter and work in mines. One governmental agency that regulates mining practice in the United States is the Mine Safety and Health Administration (“MSHA”). MSHA provides regulations as well as enforcement of the regulations.

Mining equipment is typically large and requires a significant amount of power. Mining equipment is also typically portable. To provide power to the mining equipment, portable power sources are provided in and around mines. Due to the high power requirements of mining equipment as well as mines having long shafts, often mining power source have voltages that are higher than are typically found in industrial situations. It is not uncommon for the mining power sources to provide power with voltages being about 1 kilo volt (“KV”). In addition, due to the portability of the mining equipment and power sources, often power is provided using flexible cables run without conduit. Due to the high voltages, exposed cables, wet conditions, etc., special consideration must be made for safety of the electrical power systems in mining.

One way to increase safety and reliability of mining power systems is to determine if grounding conductors are in place and properly connected. When grounding conductors are not properly connected, have short circuits, etc., when a fault condition occurs, electrical current can flow through the earth surrounding mine shafts as well as through mining equipment. Current flowing in unintended routes create a shock hazard for miners both for fault conditions before overcurrent protection reacts as well as steady state conditions when continuous current flow in unintended paths.

MSHA has provided regulations for monitoring current in grounding conductors. MSHA regulations regarding ground current were changed several years ago. Much of the electrical power system equipment in use today in mines today does not meet current MSHA requirements found in testing standard 30 of the Code of Federal Regulations (“C.F.R.”) section 75 and 30 C.F.R. section 77 with respect to ground current monitoring.

SUMMARY

An apparatus for monitoring ground current is disclosed. A system and method also perform the functions of the apparatus. The apparatus includes a signal generation module that injects a signal in a pilot conductor. The signal includes one or more alternating current (“AC”) components, where each AC component of the one or more AC components has a frequency different from other AC components of the one or more AC components. Current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, and the grounding conductor is connected to a ground return. The apparatus includes a current monitor module that monitors current in at least one of the pilot conductor and the ground return where the current is affected by a rectification circuit positioned between the pilot conductor and the grounding conductor.

The apparatus includes a DC detection module that determines a direct current (“DC”) current present in the current monitored by the current monitor module and a DC minimum threshold module that determines if the DC current is below a DC current minimum threshold. The apparatus includes an AC detection module that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module and an AC threshold module that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold. The apparatus includes a trip module that opens a contact in response to the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and/or the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.

In one embodiment, the signal injected by the signal generation module is substantially free of a DC component. In another embodiment, a carrier AC component of the one or more AC components has an amplitude that is greater than the amplitude of each of the other AC components of the one or more AC components. In another embodiment, the carrier AC component of the one or more AC components has a fundamental frequency that is lower than the fundamental frequency of each of the other AC components of the one or more AC components.

In another embodiment, the AC detection module determines the AC current corresponding to each frequency of the one or more AC components during a positive half cycle of the carrier AC component when the rectification circuit allows current. In another embodiment, the AC detection module deactivates determining a current during a negative half cycle of the carrier AC component when the rectification circuit blocks current. In another embodiment, the AC detection module concatenates signals sensed during the positive half cycle of the carrier AC component. In another embodiment, the AC detection module concatenates signals sensed during the positive half cycle of the carrier AC component by matching zero crossings of each data segment.

In one embodiment, the rectification circuit comprises one or more diodes or zener diodes. In another embodiment, the rectification circuit is connected between the pilot conductor and the grounding conductor and each diode of the one or more diodes is oriented with the anode of each of the one or more diodes connected through the pilot conductor and the cathode of each of the one or more diodes connected through the grounding conductor. In another embodiment, the DC detection module filters the affected current monitored by the current monitor module to detect the DC component.

In one embodiment, the apparatus includes a frequency change module that periodically changes a AC component characteristic of each of the one or more AC components, a frequency update module that changes a frequency sensed for each of the one or more AC components to match the fundamental frequencies of the AC components in response to the frequency change module changing the AC component characteristic of each of the one or more AC components, and a sync module that synchronizes changes in sensing by the frequency update module with changes to the fundamental frequencies of the AC components by the frequency change module.

In another embodiment, the frequency change module changes the AC component characteristic of each of the one or more AC components by changing a frequency of each of the one or more AC components and/or a phase of each of the one or more AC components. In a further embodiment, the frequency change module changes the AC component characteristic of each of the one or more AC components by ramping from a current frequency to a new frequency over a period of time, fading out a first frequency generator with a current frequency and fading in a second frequency generator with a new frequency, ramping from a current phase to a new phase over a period of time, and/or fading out a first frequency generator with a current phase and fading in a second frequency generator with a new phase.

In one embodiment, the apparatus includes a DC maximum threshold module that determines if the DC current is above a DC current maximum threshold and the trip module further opens the contact in response to the DC maximum threshold module determining that the DC current is above the DC maximum current threshold. In another embodiment, the apparatus includes a return current module that determines a current in the grounding conductor and a return current threshold module that determines if the current in the grounding conductor is below a return current threshold. In the embodiment, the trip module further opens the contact in response to the return current threshold module determining that the current in the grounding conductor is below the return current threshold for a period longer than a return current time threshold.

A system for monitoring ground current includes a power source and a ground monitor in the power source. The ground monitor includes a signal generation module that injects a signal in a pilot conductor. The signal includes one or more AC components where each AC component of the one or more AC components has a frequency different from other AC components of the one or more AC components. Current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, and the grounding conductor is connected to a ground return. The ground monitor includes a current monitor module that monitors current in at least one of the pilot conductor and the ground return where the current is affected by a rectification circuit positioned between the pilot conductor and the grounding conductor.

The ground monitor includes a DC detection module that determines a DC current present in the current monitored by the current monitor module, and a DC minimum threshold module that determines if the DC current is below a DC current minimum threshold. The ground monitor includes an AC detection module that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module and an AC threshold module that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold. The ground monitor includes a trip module that opens a contact in response to the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and/or the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold. The contact disconnecting the power source from the set of power cables.

A method for monitoring ground current includes injecting a signal in a pilot conductor, where the signal includes one or more AC components. Each AC component of the one or more AC components has a frequency different from other AC components of the one or more AC components. Current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, and the grounding conductor is connected to a ground return. The method includes monitoring current in at least one of the pilot conductor and the ground return where the current is affected by a rectification circuit positioned between the pilot conductor and the grounding conductor, determining a DC current present in the monitored current, and determining if the DC current is below a DC current minimum threshold. The method includes determining an AC current corresponding to each frequency of the one or more AC components present in the monitored current, determining if one or more of the AC currents determined by the AC detection module is below an AC threshold, and opening a contact in response to determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and/or determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.

In one embodiment, a carrier AC component of the one or more AC components has a fundamental frequency that is lower than the fundamental frequency of each of the other AC components of the one or more AC components and the carrier AC component of the one or more AC components has an amplitude that is greater than the amplitude of each of the other AC components of the one or more AC components. In another embodiment, determining the AC current of the one or more AC components includes determining the AC current corresponding to each frequency of the one or more AC components during a positive half cycle of the carrier AC component when the rectification circuit allows current and deactivating determining a current during a negative half cycle of the carrier AC component when the rectification circuit blocks current.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which:

FIG. 1 is a schematic block diagram illustrating one embodiment of a system for ground monitor current sensing in accordance with an embodiment of the present invention;

FIG. 2A is a schematic block diagram illustrating one embodiment of an apparatus for ground monitor current sensing in accordance with an embodiment of the present invention;

FIG. 2B is a schematic block diagram illustrating one embodiment of an apparatus for ground monitor current sensing with frequency hopping in accordance with an embodiment of the present invention;

FIG. 3 is a schematic block diagram illustrating another embodiment of an apparatus for ground monitor current sensing in accordance with an embodiment of the present invention;

FIG. 4 is a schematic block diagram illustrating one embodiment of a method for ground monitor current sensing in accordance with an embodiment of the present invention;

FIG. 5 is a schematic block diagram illustrating another embodiment of a method for ground monitor current sensing in accordance with an embodiment of the present invention;

FIG. 6 is a schematic block diagram illustrating one embodiment of a watchdog apparatus in accordance with an embodiment of the present invention;

FIG. 7 is a schematic block diagram illustrating another embodiment of a watchdog apparatus in accordance with an embodiment of the present invention;

FIG. 8 is a schematic block diagram illustrating a third embodiment of a watchdog apparatus in accordance with an embodiment of the present invention;

FIG. 9 is a schematic block diagram illustrating one embodiment of a method for a watchdog function in accordance with an embodiment of the present invention;

FIG. 10 is a schematic block diagram illustrating another embodiment of a method for a watchdog function in accordance with an embodiment of the present invention;

FIG. 11 is a schematic block diagram illustrating a third embodiment of a method for a watchdog function in accordance with an embodiment of the present invention;

FIG. 12 is a schematic block diagram illustrating one embodiment of a method for ground monitor current sensing with frequency hopping in accordance with an embodiment of the present invention;

FIG. 13 is a schematic block diagram illustrating another embodiment of a method for ground monitor current sensing with frequency hopping in accordance with an embodiment of the present invention; and

FIG. 14 is a diagram illustrating a generated signal with a carrier AC component and three other AC components, a rectified current, concatenated signals and determined AC components.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment, but mean “one or more but not all embodiments” unless expressly specified otherwise. The terms “including,” “comprising,” “having,” and variations thereof mean “including but not limited to” unless expressly specified otherwise. An enumerated listing of items does not imply that any or all of the items are mutually exclusive and/or mutually inclusive, unless expressly specified otherwise. The terms “a,” “an,” and “the” also refer to “one or more” unless expressly specified otherwise.

Furthermore, the described features, advantages, and characteristics of the embodiments may be combined in any suitable manner. One skilled in the relevant art will recognize that the embodiments may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments.

These features and advantages of the embodiments will become more fully apparent from the following description and appended claims, or may be learned by the practice of embodiments as set forth hereinafter. As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method, and/or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module,” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having program code embodied thereon.

Many of the functional units described in this specification have been labeled as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by various types of processors. An identified module of program code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module.

Indeed, a module of program code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. Where a module or portions of a module are implemented in software, the program code may be stored and/or propagated on in one or more computer readable medium(s).

The computer readable medium may be a tangible computer readable storage medium storing the program code. The computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, holographic, micromechanical, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing.

More specific examples of the computer readable storage medium may include but are not limited to a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a portable compact disc read-only memory (CD-ROM), a digital versatile disc (DVD), an optical storage device, a magnetic storage device, a holographic storage medium, a micromechanical storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, and/or store program code for use by and/or in connection with an instruction execution system, apparatus, or device.

The computer readable medium may also be a computer readable signal medium. A computer readable signal medium may include a propagated data signal with program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electrical, electro-magnetic, magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport program code for use by or in connection with an instruction execution system, apparatus, or device. Program code embodied on a computer readable signal medium may be transmitted using any appropriate medium, including but not limited to wire-line, optical fiber, Radio Frequency (RF), or the like, or any suitable combination of the foregoing

In one embodiment, the computer readable medium may comprise a combination of one or more computer readable storage mediums and one or more computer readable signal mediums. For example, program code may be both propagated as an electro-magnetic signal through a fiber optic cable for execution by a processor and stored on RAM storage device for execution by the processor.

Program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++, PHP or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

The computer program product may be shared, simultaneously serving multiple customers in a flexible, automated fashion. The computer program product may be standardized, requiring little customization and scalable, providing capacity on demand in a pay-as-you-go model. The computer program product may be stored on a shared file system accessible from one or more servers.

The computer program product may be integrated into a client, server and network environment by providing for the computer program product to coexist with applications, operating systems and network operating systems software and then installing the computer program product on the clients and servers in the environment where the computer program product will function.

In one embodiment software is identified on the clients and servers including the network operating system where the computer program product will be deployed that are required by the computer program product or that work in conjunction with the computer program product. This includes the network operating system that is software that enhances a basic operating system by adding networking features.

Furthermore, the described features, structures, or characteristics of the embodiments may be combined in any suitable manner. In the following description, numerous specific details are provided, such as examples of programming, software modules, user selections, network transactions, database queries, database structures, hardware modules, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that embodiments may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of an embodiment.

Aspects of the embodiments are described below with reference to schematic flowchart diagrams and/or schematic block diagrams of methods, apparatuses, systems, and computer program products according to embodiments of the invention. It will be understood that each block of the schematic flowchart diagrams and/or schematic block diagrams, and combinations of blocks in the schematic flowchart diagrams and/or schematic block diagrams, can be implemented by program code. The program code may be provided to a processor of a general purpose computer, special purpose computer, sequencer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the schematic flowchart diagrams and/or schematic block diagrams block or blocks.

The program code may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the schematic flowchart diagrams and/or schematic block diagrams block or blocks.

The program code may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the program code which executed on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

The schematic flowchart diagrams and/or schematic block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of apparatuses, systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the schematic flowchart diagrams and/or schematic block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions of the program code for implementing the specified logical function(s).

It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more blocks, or portions thereof, of the illustrated Figures.

Although various arrow types and line types may be employed in the flowchart and/or block diagrams, they are understood not to limit the scope of the corresponding embodiments. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the depicted embodiment. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted embodiment. It will also be noted that each block of the block diagrams and/or flowchart diagrams, and combinations of blocks in the block diagrams and/or flowchart diagrams, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and program code.

FIG. 1 is a schematic block diagram illustrating one embodiment of a system 100 for ground monitor current sensing in accordance with an embodiment of the present invention. The system 100 includes a ground monitor apparatus 102, a power source 104, a three-phase power source 106, a relay 108, a first phase 110, a second phase 112, a third phase 114, a neutral grounding resistor 116, a grounding conductor 118, a load 120 with a pilot wire diode (“PWD”) 124, a ground wire device (“GWD”) 126 with a return current sensor 128, a ground return 130, a ground fault relay (“GFR”) current transformer (“CT”) 132, and a watchdog apparatus 134, which are described below. The system 100 may be similar to the system 100 of U.S. patent application Ser. No. 13/906,807 entitled “GROUND MONITOR CURRENT SENSING” and filed on May 31, 2013 for Dale Curtis, et al., [hereinafter “the '807 patent application”] which is incorporated herein by reference. The '807 patent application includes a ground monitor apparatus 102 that is different than the ground monitor apparatus 102 described herein, but has similar functionality in monitoring ground current.

The system 100 includes, in one embodiment, a ground monitor apparatus 102 that monitors current in the grounding conductor 118, and is described in more detail with regard to the apparatuses 200, 201 of FIGS. 2A and 2B and the apparatus 300 of FIG. 3. The power source 104, in one embodiment, includes a three-phase power source 106. In one example, the three-phase power source 106 is a wye-connected source. The three-phase power source 106, in one example, is grounded through a neutral grounding resistor 116. In another embodiment, the three-phase power source 106 may be a delta-connected power source. In another embodiment, the power source 104 includes another type of power source, such as a single-phase power source or a direct current (“DC”) power source. The three-phase power source 106 may include some type of a generator, or maybe wired to another source (not shown). In another embodiment, the power source 104 may include multiple power sources. In a particular embodiment, the power source 104 may be configured for an application in a mine. The power source 104, in one embodiment, is configured for mining applications and in another embodiment is configured to meet requirements of the Mine Safety and Health Administration (“MSHA”). One of skill in the art will recognize other power sources 104 that may include a ground monitor apparatus 102.

In one embodiment, the power source 104 includes a relay 108. As used herein, contact and relay are used interchangeably. The relay 108 may include a contact that may be normally open or normally closed and closing or opening the contact may open the relay 108. In one example, the relay 108 is a three-phase contactor. In another example, the relay 108 is a circuit breaker with the remote trip. The relay 108 includes a capability to be commanded open by the ground monitor apparatus 102. The relay 108 may include a fault detection module (not shown) that sends a trip signal to the relay 108 on detection of an overcurrent condition. For example, a fault detection module may include current sensing and/or voltage sensing circuits may indicate an overcurrent or fault condition. The fault detection module may have an inverse time characteristic or other common protective relaying characteristic and may coordinate with other overcurrent protection devices upstream and downstream. The relay 108 may also be opened for other conditions, such as a manual command to open.

In one example, the relay 108 also includes the capability to be commanded open by the ground monitor apparatus 102. For example, the ground monitor apparatus 102 may send a command to open the relay 108 upon detection of a ground problem. In another embodiment, or the power source 104 includes a different type of power source, such as a single phase source, the relay 108 may include a different number of poles. In another example, the relay 108 may be a three-pole contactor. One of skill in the art will recognize other types of relays 108 that may be used in conjunction with the power source 104 that includes a ground monitor apparatus 102.

In one embodiment, the power source 104 includes a relay 108. In one example, the relay 108 is a three-phase contactor. In another example, the relay 108 is a circuit breaker with the remote trip. The relay 108 includes a capability to be commanded open by the ground monitor apparatus 102. The relay 108 may include a fault detection module (not shown) that sends a trip signal to the relay 108 on detection of an overcurrent condition. For example, the fault detection module may include current sensing and/or voltage sensing circuits may indicate an overcurrent or fault condition. The fault detection module may have an inverse time characteristic or other common protective relaying characteristic and may coordinate with other overcurrent protection devices upstream and downstream. The relay 108 may also be opened for other conditions, such as a manual command to open.

In one example, the relay 108 also includes the capability to be commanded open by the ground monitor apparatus 102. For example, the ground monitor apparatus 102 may send a command to open the relay 108 upon detection of a ground problem, such as a ground current different than expected. In another embodiment, or the power source 104 includes a different type of power source, such as a single phase source, the relay 108 may include a different number of poles. In another example, the relay 108 may be a three-pole contactor. One of skill in the art will recognize other types of relays 108 that may be used in conjunction with the power source 104 that includes a ground monitor apparatus 102.

In one embodiment, the system 100 includes a set of power cables wired to a load 120 with a first phase 110, a second phase 112, a third phase 114, and a grounding conductor 118. In other embodiments, the set of power cables may include a neutral conductor (grounded conductor) wired to the load and/or the pilot conductor 122. For safety, it is desirable to determine if the grounding conductor 118 is properly connected between the power source 104 and the load 120. For example, if the grounding conductor 118 is disconnected, has failed, or in some way is not properly connected between the power source 104 and the load 120, there may be a hidden danger within the system 100. For instance, if a short circuit occurs and the grounding conductor 118 is not properly connected, current may flow through the ground from the load 120 to the power source 104 and may generate an electric field within the ground that may cause an electrical hazard for personnel. In one embodiment, the ground monitor apparatus 102 helps to ensure that the grounding conductor 118 is properly connected and functioning.

In one embodiment, the ground monitor apparatus 102 injects a signal into the grounding conductor 118 to sense an undesirable condition, such as a high impedance within the grounding conductor 118, a short between the pilot conductor 122 and ground, or other failure in the grounding conductor 118 or in the ground monitor apparatus 102 and associated components.

In one embodiment, the pilot conductor 122 connects the ground monitor apparatus 102 to the PWD 124 located in the load 120. In another embodiment, an alternate PWD 136 connects the ground monitor apparatus 102 to the load 120. The PWD 124, 136, in one embodiment, includes one or more diodes or zener diodes, and the pilot conductor 122 is wired to the diodes or zener diodes. In one embodiment, the PWD 124 includes a zener diode. In the alternate embodiment, the PWD 136 is a typical diode. A terminal of the PWD 124, 136 may be wired to a chassis ground of the load 120. The grounding conductor 118, in one embodiment, is also connected to the chassis ground of the load 120. In one embodiment, the ground monitor apparatus 102 connects an alternating current (“AC”) source to the pilot conductor 122. The PWD 124, 136 may provide a voltage drop which may be used by the ground monitor apparatus 102 to determine a grounding problem. If the PWD 124, 136 fails short, if the pilot conductor 122 is shorted to ground, the current signal changes and the ground monitor apparatus 102 is able to detect the change and open the relay 108.

In an alternate embodiment, the load 120 does not include a PWD 124, 136 with a diode and the pilot conductor 122 connects to chassis ground of the load 120 and to the grounding conductor 118. In the embodiment, the ground monitor apparatus 102 may use a different current sensing scheme than where the PWD 124, 136 with a diode is included. In another embodiment, a DC voltage source injects current into the pilot conductor 122 and the load does not include a PWD 124, 136 with a diode. In another embodiment, a DC voltage source injects current into the pilot conductor 122 and the load includes a PWD 136 with one or more diodes. In the embodiment, the diode in the PWD 124 may be one or more zener diodes with the pilot conductor 122 connected to the cathode the grounding conductor 118 connected to the anode of the zener diode(s). Various embodiments will be described below with respect to the apparatuses 200, 201, 300 of FIGS. 2A, 2B and 3.

In one embodiment, the power source 104 includes a GWD 126. The GWD 126, in one embodiment, includes a return current sensor 128. In one example, the return current sensor 128 includes a current transformer that senses current in the grounding conductor 118. In another example, the grounding conductor 118 is connected to back to back diodes within the GWD 126, and the back-to-back diodes are also connected to the chassis ground of the power source 104. The back-to-back diodes, in some embodiments, provide some signal isolation for monitoring injected signal in the grounding conductor 118 from the pilot conductor 122. In addition, the back-to-back diodes in the GWD 126 help to keep any voltage on the grounding conductor 118 to within a diode drop of the chassis ground. In another embodiment, a saturable coil may replace the back-to-back diodes.

The saturable coil, in one embodiment, may help keep voltage on the grounding conductor 118 within a saturation voltage of the chassis ground. In another embodiment, the GWD 126 does not include back-to-back diodes but includes a return current sensor 128 and the grounding conductor 118 is connected to chassis ground of the power source 104. In another embodiment, the GWD 126 does not include back-to-back diodes or a return current sensor 128 and the grounding conductor 118 is connected to chassis ground of the power source 104. One skilled in the art will recognize that various other methods exist that can help limit the voltage potential between the grounding conductor 118 and the chassis ground, while providing some signal isolation for monitoring injected signal in the grounding conductor 118. In one embodiment, the system 100 includes a watchdog apparatus 134 in the ground monitor apparatus 102. The watchdog apparatus 134, in one embodiment, determines if the ground monitor apparatus 102 is functioning properly. The watchdog apparatus 134 is discussed in more detail in relation to the apparatuses 600, 700, 800 of FIGS. 6-8.

FIG. 2A is a schematic block diagram illustrating one embodiment of an apparatus 200 for ground monitor current sensing in accordance with an embodiment of the present invention. The apparatus 200 includes a pilot conductor 122, a ground return 130, and a connection to a contact of a relay 108, which are substantially similar to those described above in relation to the system 100 of FIG. 1. The apparatus 200 includes one embodiment of a ground monitor apparatus 102 with a signal generation module 202, a current monitor module 204, a direct current (“DC”) detection module 206, an alternating current (“AC”) detection module 208, a DC minimum threshold module 210, an AC threshold module 212, and a trip module 214, which are described below.

The apparatus 200 includes an embodiment of a signal generation module 202 that that injects a signal in the pilot conductor 122. In one embodiment, the signal generation module 202 connects to the pilot conductor 122 and ground return 130. In one embodiment, the signal generated by the signal generation module 202 includes a DC component and one or more AC components. The DC component may include a DC voltage that is higher than a peak voltage of the one or more AC components. For example, if an AC component includes a sinusoidal waveform with a peak voltage of 5 volts (“V”), the DC component may be 7 V. In another embodiment, the DC component is zero volts or substantially zero volts. For example, the DC component may be 0.1 V, 0.5 V, or other voltage that is substantially zero or close to zero or a voltage substantially less than a peak voltage of an AC component.

Each AC component of the one or more AC components includes a frequency different from other AC components of the one or more AC components. Current in the pilot conductor 122 is injected in the grounding conductor of the set of power cables connecting the power source 104 to the load 120 and the grounding conductor 118 of the set of power cables is connected to the ground return 130. In one embodiment, the one or more AC components are sinusoidal waveforms of different frequencies. In one embodiment, the one or more AC components are three AC components of different frequencies. In another embodiment, four or more AC components are generated by the signal generation module 202.

In one embodiment, each of the AC components generated by the signal generation module 202 has a fundamental frequency that is not a harmonic of the fundamental frequency of an AC voltage generated by the power source 104. For example, the power source 104 may generate 60 hertz (“Hz”), 120 Hz, 400 Hz, etc. Where the power source 104 produces power at 60 Hz, the AC components generated by the signal generation module 202 may be 85 Hz, 208 Hz, and 990 Hz, which are not harmonic frequencies of 60 Hz. In one embodiment, the AC components have a fundamental frequency that differs significantly from harmonic frequencies of the fundamental frequency of the power source 104. In another embodiment, one of the AC components is 60 Hz or a harmonic of 60 Hz.

The apparatus 200 includes, in one embodiment, a current monitor module 204 that monitors current in at least the pilot conductor 122 or the ground return 130 or both. For example, the current monitor module 204 may monitor current in the pilot conductor 122. In another embodiment, the current monitor module 204 monitors current in the ground return 130. In one embodiment, in normal situations current in the pilot conductor 122 is substantially the same as current in the ground return 130. In another embodiment, the current monitor module 204 monitors current in both the pilot conductor 122 and ground return 130, for example for redundancy. The current monitor module 204 may use a Hall Effect current sensor, a current transformer, a resistor, or other current sensing method known to those of skill in the art. In one embodiment, the current monitor module 204 includes current sensing capable of monitoring AC and DC current. In one embodiment, the current monitor module 204 creates a voltage representative of current in the pilot conductor 122 or ground return 130. When referring to current monitored by the current monitor module 204, one of skill in the art will recognize that the current monitor module 204 may use a voltage signal and modules using the current monitored by the current monitor module 204 may use one or more voltage signals representing current in the pilot conductor 122/ground return 130. Thus the current monitor module 204 includes a capability to monitor the DC component and the one or more AC components.

The apparatus 200, in one embodiment, includes a DC detection module 206 that determines a DC current present in the current monitored by the current monitor module 204 and an AC detection module 208 that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module 204. For example, the DC detection module 206 may determine a DC current present in the pilot conductor 122 or ground return 130. In one embodiment, the DC detection module 206 includes filtering to filter DC current from the current monitored by the current monitor module 204. For example, the DC detection module 206 may include a low pass filter that substantially removes AC content from the current monitored by the current monitor module 204. One of skill in the art will recognize other ways for the DC detection module 206 to determine DC current present in the current monitored by the current monitor module 204.

In one embodiment, the AC detection module 208 determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module 204 by filtering the DC component from the monitored current and detecting each frequency separately. For example, the AC detection module 208 may include filters to determine each frequency. In one embodiment, the AC detection module 208 uses one or more comb filters tuned to the frequencies of the power source 104. In another embodiment, the AC detection module 208 uses one or more band pass filters that pass the frequencies of the AC components while eliminating other frequencies. In another embodiment, the filters include one or more parametric filters to in conjunction with the band pass filters to further filter out unwanted frequencies while allowing the frequencies of the AC components to pass. In another embodiment, the AC detection module 208 uses one or more notch filters tuned to filter out specific unwanted frequencies. In another embodiment, the AC detection module 208 uses a combination of the above mentioned filters. For example, the AC detection module 208 may use a comb filter tuned to each frequency of the power source 104 and harmonics and may then include band pass filters and parametric filters to further isolate the frequencies of the AC components. Other embodiments may include digital signal processing (“DSP”) filtering. Resulting waveforms may then have a specific amplitude for each AC component. One of skill in the art will recognize other ways for the AC detection module 208 to determine an AC current, which may be represented as a voltage, corresponding to each frequency of the one or more AC components.

The apparatus 200, in one embodiment, includes a DC minimum threshold module 210 that determines if the DC current is below a DC current minimum threshold. For example, the DC minimum threshold module 210 may include a comparator that compares the DC current determined by the DC detection module 206 with a DC current minimum threshold. The DC minimum threshold module 210 may use other circuits as well to determine if the DC current is below a DC current minimum threshold. In another embodiment, the DC minimum threshold module 210 may include redundant circuitry and may determine if the DC current is below a first DC current minimum threshold and a second DC current minimum threshold. The first and the second DC minimum thresholds may be the same or different. Redundancy may be used to increase reliability.

In one embodiment, the DC current minimum threshold corresponds to a combined pilot conductor 122 and grounding conductor 118 resistance of less than 50 ohms. In another embodiment, the DC current minimum threshold corresponds to a combined pilot conductor 122, grounding conductor 118, and ground return 130 resistance of less than 50 ohms. Fifty ohms corresponds to an MSHA requirement and the DC current minimum threshold may be set to correspond to 50 ohms or less. For example, if the DC component is a particular voltage, the DC current minimum threshold may correspond to the DC component voltage divided by 50 ohms or a lower resistance. In one embodiment, the trip module 214 opens the contact 108 when the DC current is below the DC minimum current threshold such that if resistance of the pilot conductor 122, grounding conductor 118, and ground return exceeds a value of 50 ohms or some resistance value just under 50 ohms, the trip module 214 opens the contact 108. A first DC current minimum threshold may correspond to 45 ohms and a second DC current minimum threshold may correspond to 48 ohms.

In another embodiment, the apparatus 200 includes an AC threshold module 212 that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold. In one embodiment, the AC threshold module 212 uses a single AC threshold for each AC current corresponding to an AC component. In another embodiment, the AC threshold module 212 uses a different AC threshold for each AC current corresponding to an AC component. Note that some of the AC thresholds for the AC currents for the various AC components may be the same.

In one embodiment, the AC threshold module 212 uses a peak current to compare to an AC threshold. In another embodiment, the AC threshold module 212 uses a root-mean-square (“RMS”) of the current to compare to an AC threshold. The AC threshold module 212 may use one or more comparators or circuits with a similar function to determine if one or more of the AC currents is below one or more AC thresholds. In another embodiment, the AC current may be digitized and the AC threshold module 212 may digitally compare one or more of the AC currents to one or more AC thresholds. In another embodiment, the AC threshold module 212 includes redundant circuitry so that for each current corresponding to an AC component, there are at least two comparators or similar circuitry. One of skill in the art will recognize other ways for the AC threshold module 212 to determine if one or more of the AC currents is below an AC threshold.

The apparatus 200, in one embodiment, includes a trip module 214 that opens a contact 108 in response to the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and/or the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold. The contact 108 disconnects the power source 104 from the set of power cables. Note that use of the phrase “opens a contact 108” may include closing a normally open contact and/or opening a normally closed contact that is part of the relay 108 such that the power source 104 is disconnected from the set of power cables to the load 120.

The DC current minimum time threshold may be zero or substantially zero or may be an appreciable amount of time. For example, the DC current minimum time threshold may be set to zero while a time may elapse between sending a signal and actual opening due to typical circuit delay time while the delay is unintentional. In another embodiment, the DC current minimum time threshold may be set to a value that accounts for transient conditions, motor start times, downstream overcurrent device reaction time, or other delay known to those of skill in the art. Likewise the AC current time threshold may be zero, substantially zero, or another time and each AC component may have the same or a different AC current time threshold.

In one embodiment, the trip module 214 opens the contact 108 in response to one of the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and the AC threshold module determining that an AC current is below an AC threshold for longer than an AC current time threshold. In another embodiment, the trip module 214 opens the contact 108 in response to some combination of the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and the AC threshold module determining that at least one or more of the AC currents is below an AC threshold for longer than an AC current time threshold.

In one embodiment, the apparatus 200 includes multiple DC current minimum thresholds and multiple AC current minimum thresholds for each AC component and the trip module 214 reacts differently depending upon which threshold or thresholds are crossed. For example, certain AC or DC current minimum thresholds may cause the trip module 214 to open the contact 108 without any other condition where other AC or DC current minimum thresholds may require some combination of threshold crossings to open. In other embodiments, certain AC or DC current minimum thresholds may merely trigger an alert while others may cause the trip module 214 to open the contact 108.

In one embodiment, the trip module 214 opens the contact 108 in response to the AC threshold module 212 determining that for at least two of the AC currents, an AC current is below an AC threshold for longer than a first AC current time threshold. In a further embodiment, the signal generation module 202 generates three AC components and the trip module 214 further opens the contact 108 in response to the AC threshold module 212 determining that each of the three AC currents is below an AC threshold for longer than a second AC current time threshold. In one embodiment, the first AC current time threshold is longer than the second AC current time threshold. For example, the first AC current time threshold may be 250 milliseconds (“mS”) and the second AC current time threshold may be 150 mS.

In another embodiment, the signal generated by the signal generation module 202 does not include a DC component and includes one or more AC components. In the embodiment, the alternate PWD 136 may be used to affect or rectify the signal from the signal generation module 202. The signal generated by the signal generation module 202 may be substantially free of a DC component. The term “substantially free of a DC component” hereinafter includes a signal that does not intentionally include a DC component but may include a small or negligible DC component unintentionally present. In the embodiment, the current monitor module 204 monitors current in the pilot conductor 122 and/or the ground return 130 where the current is rectified with or at least affected by a rectification circuit positioned between the pilot conductor 122 and the grounding conductor 130. The rectification circuit, in one embodiment, is the alternate PWD 136.

In various embodiments, the rectification circuit may include one or more diodes connected in series or parallel where each diode of the one or more diodes is oriented with the anode of each of the one or more diodes connected through the pilot conductor 122 and the cathode of each of the one or more diodes connected through the grounding conductor 130. For example, the rectification circuit may include two diodes in series or parallel each oriented as depicted in FIG. 1, may include four diodes with two in series and each pair of diodes connected in parallel, each oriented as depicted in FIG. 1, etc. Other devices may also be used in the rectification circuit that provide a rectification function. One of skill in the art will recognize other devices and ways to provide a rectification function for a PWD 136.

In one embodiment, the DC detection module 206 may detect the DC component present in the current monitored by the current monitor module 204. In one embodiment, the DC detection module 206 filters the affected current monitored by the current monitor module 204 to detect the DC component. For example, the DC detection module 206 may include a low pass filter or other device to isolate the DC component from other frequencies present in the monitored current.

In one embodiment, the signal generation module 202 includes a carrier AC component of the one or more AC components that has a fundamental frequency that is lower than the fundamental frequency of each of the other AC components of the one or more AC components. The AC detection module 208 may detect the carrier AC component along with the other AC components or may ignore the carrier AC component and may then detect the other AC components. In one embodiment, the carrier AC component has a fundamental frequency that matches the fundamental frequency of the power source 104 (e.g. 60 Hz). In another embodiment, the carrier AC component of the one or more AC components has an amplitude that is greater than the amplitude of each of the other AC components of the one or more AC components. For example, the carrier AC component may have an amplitude that is much larger than the amplitudes of the other AC components so that the affected monitored current has a significant DC component. A larger amplitude for the carrier AC component may also help lessen effects of rectification or other diode effects so that the DC component present in the rectified carrier AC component is larger than voltage drop across the PWD 136.

In one embodiment, the AC detection module 208 determines the AC current corresponding to each frequency of the one or more AC components during a positive half cycle of the carrier AC component when the rectification circuit allows current. For example, if the PWD 136 is oriented as depicted in FIG. 1, a positive half cycle of the carrier AC component will conduct through the diode so that a significant amount of current is transmitted through the PWD 136, pilot conductor 122, and ground return 130.

In another embodiment, the AC detection module 208 deactivates determining a current during a negative half cycle of the carrier AC component when the rectification circuit blocks current. During the negative half cycle of the carrier AC component, the PWD 136 blocks current so that only a relatively small amount of current passes through the PWD 136, pilot conductor 122, and ground return 130. The AC detection module 208 may ignore the monitored current during the negative half cycle of the carrier AC component. For example, the AC detection module 208 may include a digital signal processing (“DSP”) filter that can be programmed to ignore certain time periods. In a further embodiment, the AC detection module concatenates signals sensed during the positive half cycle of the carrier AC component. The concatenated current signals may include a discontinuity where one positive half wave signal is joined to another positive half wave signal. In one example, the AC detection module 208 tolerates the discontinuity and detects the one or more AC components. In another embodiment, the AC detection module 208 ignores the discontinuity, for example by deactivating sensing just prior to and just after the discontinuity. In another embodiment, the AC detection module 208 determines zero crossings near the beginning and end of the monitored current of a positive half of the carrier AC component and joins the positive half wave monitored current segments at the zero crossing points. This zero crossing approach may minimize any discontinuity at the points where waveforms are joined.

In another embodiment, the signal generated by the signal generation module 202 includes AC components without a carrier AC component. In the embodiment, the AC detection module 208 determines each AC component from the rectified current monitored by the current monitor module 202. The AC components, in the embodiment, may each have an amplitude higher or significantly higher than a voltage drop across the PWD 136. Various techniques may be used to determine the AC components, such as comb filters, band pass filters, parametric filters, and DSP filters. The DSP filters may use a fast Fourier transform or another similar technique to derive the various AC components from the affected current monitored by the current monitor module 204.

For embodiments with and without a carrier AC component, the signal generation module 202 may adjust an amplitude of each AC component and the AC threshold module 212 may adjust the one or more AC thresholds to account for rectification and associated voltage drop across the PWD 136. One of skill in the art will recognize how to adjust amplitudes and associated AC thresholds based on the affected current.

FIG. 14 is a diagram illustrating a generated signal with a carrier AC component and three other AC components, a rectified current, concatenated signals and determined AC components. Note that the graphs in FIG. 14 are merely illustrative and are not intended to accurately depict actual waveforms. The top graph of FIG. 14 illustrates a carrier AC component that includes three other AC components of higher frequencies and lower amplitudes than the carrier AC component. The top graph may represent an injected signal by the signal generation module 202. The second graph illustrates a rectified current that may represent a current monitored by the current monitor module 204. Note that the graph depicts a significant amount of current during each positive half wave of the carrier AC component and does not include any current during the negative half wave of the carrier AC component. Note that a PWD 136 with a diode will include some voltage drop that would alter the rectified current. As the amplitude of the carrier AC waveform increases relative to the voltage drop of the PWD 136, the effect of the voltage drop is minimized. Note that other diode types and diode configurations may affect the current in different ways, however, a DC component is typically present in the affected current.

The third graph illustrates concatenating of the positive half wave signals. The AC detection module 208 may include a DSP filter to perform the concatenation and to filter out the AC components. The fourth graph illustrates three AC components that may be determined by the AC detection module 208 that may be used by the AC threshold module 212 to compare to one or more AC thresholds. Note that the carrier AC component is not sensed by the AC detection module 208 in this example.

FIG. 2B is a schematic block diagram illustrating one embodiment of an apparatus 201 for ground monitor current sensing with frequency hopping in accordance with an embodiment of the present invention. The apparatus 201 includes a pilot conductor 122, a ground return 130, and a connection to a contact of a relay 108, which are substantially similar to those described above in relation to the system 100 of FIG. 1. The apparatus 201 includes one embodiment of a ground monitor apparatus 102 with a signal generation module 202, a current monitor module 204, an alternating current (“AC”) detection module 208, an AC threshold module 212, and a trip module 214, which are substantially similar to those described above in relation to the apparatus 200 of FIG. 2A.

In addition, the apparatus 201 includes a frequency change module 216 that periodically changes a AC component characteristic of each of the one or more AC components. For example, if the signal generation module 202 injects a signal in the pilot conductor 122 that includes one AC component, the frequency change module 216 may periodically change the fundamental frequency of the AC component. Changing the AC component characteristic, in one embodiment, includes changing the frequency of the AC component. For example, if the AC component is 990 Hz, the frequency change module 216 may change the frequency to 900 Hz. The AC detection module 208 first detects the 990 Hz and then detects the 900 Hz after the frequency change. The frequency change module 216 changing the frequency may be called frequency hopping or frequency hopping spread spectrum (“FHAA”).

In one embodiment, the frequency change module 216 changes the frequency of the AC component over a range or bandwidth. In another embodiment, the frequency change module 216 changes the frequency of the AC component multiple times during a cycle of the fundamental frequency of the power source 104. For example, if the power source 104 transmits power at 60 Hz, the period of a cycle is 16.7 milliseconds (“mS”) and the frequency change module 216 changes the frequency of the AC component multiple times during the 16.7 mS. In another embodiment, the frequency change module 216 changes the frequency of the AC component less frequent than the fundamental frequency of the power source 104. For example, the frequency change module 216 may change the frequency of the AC component several times a second. In another embodiment, the frequency change module 216 may change the frequency of the AC component at a rate slower than action by the AC threshold module 212, the DC minimum threshold module 210, the trip module 214 and/or other overcurrent protection elements.

In another example, signal generation module 202 generates two more AC components and the frequency change module 216 changes the frequency of each of the AC components. In one example, the frequency change module 216 changes the AC component characteristic by changing the fundamental frequency of each of the one or more AC components between frequencies of a predefined group of frequencies. In another example, the frequency change module 216 changes the frequency of each of the AC components over a range that doesn't overlap other of the AC components. For example, if the signal generation module 202 injects a signal with three AC components that are initially 85 Hz, 208 Hz, and 990 Hz, the frequency ranges may be 80-100 Hz, 190-250 Hz, and 850-1050 Hz. In a related embodiment, the frequency change module 216 may avoid harmonics of the fundamental frequency of the power source 104, which may avoid noise issues from harmonic content caused by equipment powered by the power source 104 or by the power source 104 itself. In another embodiment, the frequency change module 216 may only select frequencies such that the frequencies are harmonically unrelated to one another.

In another embodiment, changing the AC component characteristic includes changing a phase of each of the one or more AC components. For example, the signal generation module 202 may generate a fixed frequency for each of the one or more AC components injected in the pilot conductor 122 and the frequency change module 216 may periodically change the phase of each of the AC components. The change in phase may be relative to a particular reference, such as zero crossing of a frequency such as the fundamental frequency of the power source 104 or a reference with respect to each AC component. In one embodiment, the frequency change module 216 includes a delay buffer that changes the phase of each of the one or more AC components. Each AC component may have a separate delay buffer. The frequency change module 216 may change the phase of each of the AC components at a rate faster than the fundamental frequency of the power source 104.

Using the frequency change module 216 to change the fundamental frequency of each of the AC components, in one embodiment, helps to minimize interference with a stray frequency component present in the current measured by the current monitor module 204 Changing the phase and/or frequency of each AC component faster than the AC threshold module 212, the DC minimum threshold module 210, the trip module 214 and/or other overcurrent protection elements may help to further reduce interference with stray frequencies because any frequency chosen by the frequency change module 216 will only have interference for a short period of time relative to the AC threshold module 212, the DC minimum threshold module 210, the trip module 214 and/or other overcurrent protection elements.

One source of stray frequencies may be other ground monitors that may also inject a signal in their respective pilot conductors that include AC components, which may then inject the AC components in a common ground system or power system. The other ground monitors may also include a frequency change module and may at times generate a frequency that is the same as the AC components injected by the signal generation module 202. In one embodiment, where other ground monitors include a ground monitor apparatus 102, the frequency change module 216 of each ground monitor may choose frequencies or phase shifts different than each other. In another embodiment, each frequency change module 216 may change frequencies randomly. In another embodiment, each frequency change module 216 may change frequencies in a group of frequencies and each ground monitor may have a different group of frequencies. One of skill in the art will recognize other ways to minimize interference between ground monitors that inject one or more AC components.

FIG. 3 is a schematic block diagram illustrating another embodiment of an apparatus 300 for ground monitor current sensing in accordance with an embodiment of the present invention. The apparatus 300 includes a pilot conductor 122, a ground return 130, a connection to a contact of a relay 108, a grounding conductor 118, a return current sensor 128, and a GFR CT 132, which are substantially similar to those described above in relation to the system 100 of FIG. 1 and one embodiment of a ground monitor apparatus 102 with a signal generation module 202, a current monitor module 204, a direct current (“DC”) detection module 206, an alternating current (“AC”) detection module 208, a DC minimum threshold module 210, an AC threshold module 212, and a trip module 214, which are substantially similar to those described in relation to the apparatus 200 of FIGS. 2A and 2B. The apparatus 300 includes, in various embodiments, a DC maximum threshold module 302, a return current module 304, a return current threshold module 306, a ground fault current module 308, a ground fault threshold module 310, a filter module 312, a frequency update module 314, a sync module 316, one or more oscillators 320, and one or more delay buffers 318, which are described below.

The apparatus 300, in one embodiment, includes a DC maximum threshold module 302 that determines if the DC current, determined by the DC detection module 206, is above a DC current maximum threshold. In one embodiment, the trip module 214 also opens the contact 108 in response to the DC maximum threshold module 302 determining that the DC current is above the DC maximum current threshold. In some circumstances, the grounding conductor 118 or other circuit of the system 100 may include stray DC voltage and/or current that may increase the DC current component of the current in the pilot conductor 122 or ground return 130 monitored by the current monitor module 204 and the DC detection module 206 may determine that the DC current is above a DC current maximum threshold. In one example, the DC current maximum threshold is set at a level that corresponds to a current that is above a DC current in the pilot conductor 122 or ground return 130 that is at a level indicative of an operating condition without a stray DC current component.

In one embodiment, the PWD 124 includes one or more diodes such that the PWD 124 has a particular voltage drop when the signal is applied to the pilot conductor 122. For example, the PWD 124 may include one or more zener diodes with a specific breakdown voltage. In another example, the alternate PWD 136 includes one diode or two or more diodes in series oriented to conduct current injected in the pilot conductor 122 oriented with the anodes connected (directly or through another diode) to the pilot conductor 124. The diodes include a voltage drop. A number of diodes may be used to create a desired voltage drop across the PWD 124 during normal operation. In one embodiment, the DC current maximum threshold corresponds to a current that is above a DC current in the pilot conductor 122 or ground return 130 when the load 120 includes a properly functioning PWD 124. For example, if the voltage drop across the PWD 124 is 5 V, the DC current maximum threshold may be 7 V.

The apparatus 300, in one embodiment, includes a return current module 304 that determines a current in the grounding conductor 118, and a return current threshold module 306 that determines if the current in the grounding conductor 118 is below a return current threshold. In the embodiment, the trip module 214 also opens the contact 108 in response to the return current threshold module 306 determining that the current in the grounding conductor 118 is below the return current threshold for a period longer than a return current time threshold. In one example, the return current module 304 includes a return current sensor 128 positioned to determine current in the grounding conductor 118.

The return current sensor 128, in one embodiment, is a current transformer. The return current sensor 128, in one embodiment, is in the GWD 126 and monitors current before the grounding conductor 118 connects to one or more diodes connected to a chassis ground. In another embodiment, return current sensor 128 is a Hall-Effect sensor. In another embodiment, the return current sensor 128 includes a resistor and voltage across the resistor is proportional to current in the grounding conductor 118. One of skill in the art will recognize other types of return current sensors 128 capable of measuring current in the grounding conductor 118. The return current module 304 may also include other circuitry, such as resistors, capacitors, op amps, etc. known to those of skill in the art.

In one embodiment, the return current threshold may be a value that represents current just below a current level for the signal injected in the pilot conductor 122 during typical operation. In a circumstance where the signal generation module 202 injects a signal and current in the grounding conductor 118 is below an expected current, which is above the return current threshold, this condition may represent a situation where there is a break or higher impedance somewhere in the circuit formed by the pilot conductor 122, the PWD 124, 136 (where included), and the grounding conductor 118 and all or a portion of the current of the signal injected by the signal generation module 202 may be returning through an alternate ground path. In one embodiment, the return current time threshold is zero or substantially zero, for example where the return current time threshold is set to not include any intentional delay. In another embodiment, the return current threshold is set to a value greater than zero. The return current threshold may be set to a value longer than expected transients or other condition where opening the contact 108 may be undesirable. One of skill in the art will recognize other appropriate values for the return current time threshold.

In one embodiment, the apparatus 300 includes a ground fault current module 308 that determines a ground fault current. In one embodiment, the ground fault current is measured in a connection between a ground of the power source 104 and a chassis ground of the power source 104. For example, the power source 104 may include a connection to chassis ground and the ground fault current module 308 may determine current in the connection. In one example, the ground connection of the power source 104 may include a neutral grounding resistor 116. The ground fault current module 308 may monitor current through the neutral grounding resistor 116 or in conductors between a ground of the power source 104 and a chassis ground. In one embodiment, the chassis ground is also an earth ground. The neutral grounding resistor 116, in some embodiments, is sized to limit current during a fault condition to below a specified value. For example, where the power source 104 is for mining equipment, the neutral grounding resistor 116 may be sized to limit current during a fault to a level specified by MSHA.

In one embodiment, the ground fault current module 308 measures current by measuring voltage across the neutral grounding resistor 116. In another embodiment, the ground fault current module 308 measures current using a current transformer, such as the GFR CT 132 depicted in FIG. 1. In another embodiment, the ground fault current module 308 includes other circuitry to provide a signal useful to the ground fault threshold module 310, such as a voltage proportional to current in the connection between the ground of the power source 104 and the chassis ground. In another embodiment, the ground fault current module 308 measures ground fault current from the first phase 110, the second phase 112, and the third phase 114. For example, a three phase load may be balanced so that the phase currents are equal and any difference in phase current may cause current to flow to the ground or chassis ground, possibly through a neutral grounding resistor 116. A GFR CT 132 may be positioned around the phase conductors 110, 112, 114 and differential current causes the GFR CT 132 to sense ground fault current. One of skill in the art will recognize other ways to measure ground fault current.

The apparatus 300 includes, in one embodiment, a ground fault threshold module 310 that determines if the ground fault current determined by the ground fault current module 308 is above a ground fault threshold. The trip module 214 also opens the contact 108 in response to the ground fault threshold module 310 determining that the current determined by the ground fault current module 308 is above the ground fault threshold for a period longer than a ground fault time threshold. In one embodiment, the ground fault threshold is zero or substantially zero, for example where the ground fault threshold is set to zero without any intentional delay. In another embodiment, the ground fault threshold is set to a value higher than zero. The ground fault threshold may be used to coordinate with other ground fault trip settings of downstream overcurrent protection devices, may be set to account for acceptable transient conditions, etc. The return current threshold module 306 and the ground fault threshold module 310 may include comparators, op amps, etc. to compare a current signal with a threshold.

In one embodiment, the apparatus 300 includes a filter module 312 that that separates each of the DC component and the one or more AC components from each other. For example, the filter module 312 may include the DC detection module 206 and the AC detection module 208 and may include a comb filter, band pass filters, parametric filters, etc. as mentioned above in relation to the DC detection module 206 and the AC detection module 208. In one embodiment, the filter module 312 substantially removes at least one of a power source fundamental AC component and the harmonics of the power source fundamental AC component from within the one or more AC components of the current monitored by the current monitor module 204. For example, the fundamental AC component of the power source 104 may be 60 Hz and the filter module 312 may remove 60 Hz and harmonics of 60 Hz.

In one embodiment, the apparatus 300 includes a frequency update module 314, which may be part of the AC detection module 208, that that changes a frequency sensed for each of the one or more AC components to match the fundamental frequencies of the AC components in response to the frequency change module 216 changing the AC component characteristic of each of the one or more AC components. For example, when the frequency change module 216 changes a frequency of an AC component to a particular frequency, the frequency update module 314 may change the AC detection module 208 to detect the particular frequency. In one example, the signal generation module 202 may include an oscillator 320 for each AC component where the frequency generated by each oscillator 320 is adjustable. The frequency change module 216 may adjust the oscillator 320 to change an AC component from a current frequency to a new frequency. The frequency update module 314 may then change the AC detection module 208 and/or elements in the filter module 312 to sense the new frequency. For example, the frequency update module 314 may update coefficients for one or more filters associated with an AC component to sense the new frequency. For example, the filters may be digital signal processing (“DSP”) filters where sensed frequencies may be changed by changing coefficients.

Where the frequency change module 216 changes a phase of the one or more AC components from a current phase to a new phase, the frequency update module 314 may adjust the AC detection module 314 and/or filter module 312 for each AC component to sense the new phase. For example, the signal generation module 202 may include one or more delay buffers 318 that shift the phase of each AC component. The frequency update module 314 may change the sensed phase shift from a current phase shift to a new phase shift for each of the AC components.

In one embodiment, the apparatus 300 includes a sync module 316 that synchronizes changes in sensing by the frequency update module 314 with changes to the fundamental frequencies of the AC components by the frequency change module 216. For example, the sync module 316 may include communication between the frequency change module 216 and the frequency update module 314 so that changes to frequencies initiated by the frequency change module 216 are communicated to the frequency update module 314. In one embodiment, the signal generation module 202, AC detection module 208, frequency change module 216, frequency update module 314 and sync module 316 are located together in a single electronic device, such as a chip, a controller, a PC board, etc. so that the sync module 316 is implemented internal to the electronic device. In another embodiment, the sync module 316 uses a communication pathway, such as one or more signal traces, a network connection, a wireless connection, etc.

Abruptly changing the frequency of one or more AC components may result in a transient. The apparatus may include one or more methods of reducing transients caused by the frequency change module 216 changing frequencies or mitigating the effects of the transient. For example, the frequency change module 216 may change the AC component characteristic of each of the one or more AC components by ramping from a current frequency to a new frequency over a period of time. For example, if a current frequency of an AC component is 990 Hz and the frequency change module 216 changes to 950 Hz, the frequency change module 216 may ramp the frequency of the AC component from 990 to 950 Hz through intermediate frequencies. In another embodiment where the frequency change module 216 changes phase of each AC component, the frequency change module 216 may ramp from a current phase to a new phase over a period of time, for example by changing phase over a range through intermediate phases.

In one embodiment, the frequency change module 216 changes the fundamental frequency of each of the one or more AC components randomly. For example, the frequency change module 216 may include a random number generator that generates a random number within a range and the frequency change module 216 calculates a corresponding new frequency within a range of frequencies available for an AC component. The frequency change module 216 may use the same technique to change each AC component simultaneously or separately. In one embodiment, the frequency change module 216 includes at least one oscillator 320 for each AC component. Each oscillator 320 is capable of being adjusted to output various frequencies. For example, the oscillator 320 may be an unstable oscillator that is controlled to generate a particular frequency. In one embodiment, an oscillator 320 is set to a frequency using a coefficient and the frequency change module 216 generates a random frequency for each of the one or more an AC components and calculates coefficients for an oscillator 320 corresponding to each AC component to generate the random frequency for each of the one or more AC components. The AC detection module 208 then calculates coefficients for one or more filters 312 to match the random frequency for each of the one or more AC components. One of skill in the art will recognize other ways to generate a random frequency for each AC component.

In another embodiment, the frequency change module 216 includes a first and a second frequency generator, such as an oscillator 320, for each of the one or more AC components and changes, for an AC component, from a current frequency generated by the first frequency generator to a new frequency generated by the second frequency generator. Each first frequency generator fades out the current frequency and each second frequency generator fades in the new frequency. Inclusion of multiple frequency generators for a particular AC component adds cost, but may minimize transients during a frequency change.

In another embodiment, the AC detection module 208 deactivates detecting the AC current of the one or more AC components during a time period when the frequency change module 216 changes the AC component characteristic by changing the fundamental frequency and/or phase of each of the one or more AC components. The time period of deactivation of detection may correspond to a transient period after a frequency change. Where an abrupt frequency change causes a transient that is unacceptable, due to length of time to settle or other negative characteristic, the frequency change module 216 may employ one or more of the techniques described above to ramp changes to minimize transients and negative effects resulting therefrom. In one embodiment, the period of time between when the frequency change module 216 changes frequency is affected by transients, ramping, etc. and is typically a factor in determining a minimum time between frequency changes.

FIG. 4 is a schematic block diagram illustrating one embodiment of a method 400 for ground monitor current sensing in accordance with an embodiment of the present invention. The method 400 begins and injects 402 a signal in a pilot conductor 122. The signal includes a DC component and one or more AC components. Each AC component has a frequency different from other AC components. As depicted in FIG. 1, current in the pilot conductor 122 is injected in the grounding conductor 118 of a set of power cables connecting the power source 104 to the load 120. Also, the grounding conductor 118 connects to the ground return 130. In one embodiment, the signal generation module 202 injects 402 the signal in the pilot conductor 122. The method 400 monitors 404 current in the pilot conductor 122, the ground return 130, or both. The current monitor module 204, in one embodiment, monitors 404 the current. The method 400 determines 406 a DC current present in the monitored current and determines 408 if the DC current is below a DC current minimum threshold. In one embodiment, the DC detection module 206 determines the DC current present in the current monitored 404 by the current monitor module 204 and the DC minimum threshold module 210 determines 408 if the DC current is below a DC current minimum threshold.

If the method 400 determines 408 that the DC current is not below a DC current minimum threshold, the method 400 returns and continues to inject 402 a signal in the pilot conductor 122. If the method 400 determines 408 that the DC current is below a DC current minimum threshold, the method 400 opens 410 the contact 108, and the method 400 ends. The method 400 determines 412 an AC current corresponding to each frequency of the one or more AC components present in the monitored current and determines 414 if one or more of the AC currents is below an AC threshold. If the method 400 determines 414 that one or more of the AC currents is not below an AC threshold, the method 400 returns and continues to inject 402 a signal in the pilot conductor 122. If the method 400 determines 414 that one or more of the AC currents is below an AC threshold, the method 400 opens 410 the contact 108, and the method 400 ends. The DC detection module 206, the AC detection module 208, the DC minimum threshold module 210, and the AC threshold module 212 may be used in determining DC and AC components and if the DC and AC components are below thresholds.

FIG. 5 is a schematic block diagram illustrating another embodiment of a method 500 for ground monitor current sensing in accordance with an embodiment of the present invention. The method 500 begins and injects 502 a signal in a pilot conductor 122. The signal includes a DC component and one or more AC components. Each AC component has a frequency different from other AC components. The method 500 monitors 504 current in the pilot conductor 122, the ground return 130, or both. The method 500 determines 506 a DC current present in the monitored current and determines 508 if the DC current is below a DC current minimum threshold. If the method 500 determines 508 that the DC current is below a DC current minimum threshold, the method 500 opens 512 the contact 108. If the method 500 determines 508 that the DC current is not below a DC current minimum threshold, the method 500 determines 510 if the DC current is above a DC current maximum threshold. If the method 500 determines 510 that the DC current is not above the DC current maximum threshold, the method 500 returns and continues to inject 502 a signal in the pilot conductor 122. If the method 500 determines 510 that the DC current is above the DC current maximum threshold, the method 500 opens 512 the contact 108. The DC maximum threshold module 302 may be used to determine 510 if the DC current is above the DC current maximum threshold.

The method 500 determines 514 an AC current corresponding to each frequency of the one or more AC components present in the monitored current and determines 516 if one or more of the AC currents is below an AC threshold. If the method 500 determines 516 that one or more of the AC currents is not below an AC threshold, the method 500 returns and continues to inject 502 a signal in the pilot conductor 122. If the method 500 determines 516 that one or more of the AC currents is below an AC threshold, the method 500 opens 512 the contact 108.

The method 500 determines 518 a current in the grounding conductor 118 and determines 520 if the current in the grounding conductor 118 is below a return current threshold. If the method 500 determines 520 that the current in the grounding conductor 118 is not below the return current threshold, the method 500 returns and continues to inject 502 a signal in the pilot conductor 122. If the method 500 determines 520 that the current in the grounding conductor 118 is below the return current threshold, the method 500 opens 512 the contact 108. The return current module 304 and the return current threshold module 306 may be used to determine 518 current in the grounding conductor 118 and to determine 520 if the current in the grounding conductor 118 is below a return current threshold.

The method 500 determines 522 a ground fault current and determines 524 if the ground current is above a ground fault threshold. If the method 500 determines 524 that the ground fault current is not above a ground fault threshold, the method 500 returns and continues to inject 502 a signal in the pilot conductor 122. If the method 500 determines 524 that the ground fault current is above a ground fault threshold, the method 500 opens 512 the contact 108, and the method 500 ends. The ground fault current module 308 and the ground fault threshold module 310 may be used to determine 522 a ground fault current and to determine 524 if the ground current is above a ground fault threshold.

In one embodiment, the method 500 of FIG. 5 does not determine 506 a DC current present in the monitored current, does not determine 508 if the DC current is below a DC current minimum threshold, or if the DC current is above the DC current maximum threshold, but continues to perform the other functions depicted in FIG. 5.

FIG. 6 is a schematic block diagram illustrating one embodiment 600 of a watchdog apparatus 134 in accordance with an embodiment of the present invention. The watchdog apparatus 134 includes a watchdog signal module 602, a signal monitoring module 604, and an alert module 606, which are described below. In one embodiment, the watchdog apparatus 134 determines if a device, such as a microprocessor or a device capable of generating a signal, is functioning properly. The device may be part of a computing device, a smartphone, an appliance, or any other device where monitoring functionality of a microprocessor or similar device would be beneficial. In one embodiment, the watchdog apparatus 134 is part of the ground monitor apparatus 102, and the watchdog apparatus 134 monitors functionality of the ground monitor apparatus 102. In one embodiment, the ground monitor apparatus 102 with the watchdog apparatus 134 complies with MSHA testing standard 30 the Code of Federal Regulations (“C.F.R.”) section 75, 30 C.F.R. section 77, American Society for Testing and Materials (“ASTP”) standard 2135, and/or MSHA document number ACRI 2003. Typical ground monitors using microprocessors or similar technologies fail to comply with these standards. The ground monitor apparatus 102 described herein is designed to meet these standards.

In one embodiment, the watchdog apparatus 134 includes a watchdog signal module 602 that generates a watchdog signal. The watchdog signal is generated at a specific interval and includes an identifiable characteristic. The watchdog signal module is part of a device being monitored for functionality. For example, the identifiable characteristic may be a pulse with a particular pulse width. The pulse may occur at a specific interval or frequency. In one embodiment, each pulse includes the same identifiable characteristic. In another example, the identifiable characteristic is a digital pattern and the digital pattern is generated periodically.

In another embodiment, the identifiable characteristic includes a pulse that fits within a specific frequency range. Additional frequencies could be included with a carrier frequency. In another embodiment, the identifiable characteristic includes a pulse with a particular pattern of highs and lows (i.e. ones and zeros) that is periodically broadcast. In another embodiment, the identifiable characteristic includes an analog signal coming from a digital-to-analog converter (“DAC”) with a specific, measurable characteristic, such as multiple tones at different frequencies. One of skill in the art will recognize other identifiable characteristics that may be included in a signal that may be used to determine that a device is functioning.

In one embodiment, the watchdog signal module 602 is included within a device being monitored while the signal monitoring module 604 is external to the monitored device but internal to a device controlled and/or powered by the monitored device. For example, the monitored device may be a microprocessor in a computer and the watchdog signal module 602 is in the microprocessor while the signal monitoring module 604 and possibly the alert module 606 is within the computer and external to the microprocessor. In another embodiment, the watchdog signal module 602 is in a processor in the ground monitor apparatus 102 while the signal monitoring module 604 and at least a portion of the alert module 606 are external to the processor but within the ground monitor apparatus 102.

In one embodiment, the watchdog apparatus 134 includes a signal monitoring module 604 that determines if the watchdog signal includes the identifiable characteristic. For example, if the identifiable characteristic is a pulse of a specific width, the signal monitoring module 604 can determine if the pulse width is as expected. If the watchdog signal includes specific frequencies, the signal monitoring module 604 can determine if the specific frequencies are present. If the watchdog signal is a digital pattern, the signal monitoring module 604 can determine if the digital pattern is present in the watchdog signal. Typically, if the monitored device is malfunctioning, there is a good chance that the watchdog signal will vary from the identifiable characteristic.

In one embodiment, the watchdog apparatus 134 includes an alert module 606 that sends and alert signal in response to the signal monitoring module 604 determining that the watchdog signal does not include the identifiable characteristic. The alert signal may include a message to a particular device. In another embodiment, the alert signal includes triggering an action, such as a reset or a shutdown of the monitored device or the device that includes the modules 602-606. In another embodiment, the alert signal is used to trigger an action external to the device that includes the modules 602-606. For example, if the device is the ground monitor apparatus 102, the alert signal may be a trip signal that opens the contact 108 to disconnect the power source 104 from the load 120. In another embodiment, the alert signal may include one or more messages, notifications, etc. as well as triggering some action like opening the contact 108 or shutting down a processor.

FIG. 7 is a schematic block diagram illustrating another embodiment 700 of a watchdog apparatus 134 in accordance with an embodiment of the present invention. The embodiment 700 of the watchdog apparatus 134 in a ground monitor apparatus 102 and the watchdog apparatus 134 includes a watchdog signal module 602 with a first pulse module 702, a signal monitoring module 604 with a first pulse average module 704 and a first pulse window module 706, and an alert module 606, which is substantially similar to the alert module 606 described in relationship with the embodiment 600 of FIG. 6. In various embodiments, the watchdog apparatus 134 may also include a second pulse module 708, a second pulse average module 710, and a second pulse window module 712, which are described below.

In one embodiment, an embodiment 700 of the watchdog apparatus 134 includes a watchdog signal module 602 with a first pulse module 702 that generates a first pulse signal of a first frequency. The first pulse signal includes a pulse of a first pulse width. The watchdog signal includes, in this embodiment, the first pulse signal. The first pulse module 702, in one embodiment, is included in a device that is being monitored by the watchdog apparatus 134. For example, the first pulse module 702 may be included in a processor. The processor, in one embodiment, is in the ground monitor apparatus 102.

The first pulse includes a first pulse width that is typically less than the period of the first frequency. The first pulse module 702 generates a first pulse at the first frequency. For example, if the first frequency is 10 kilohertz (“kHz”), the period for this frequency is 100 microseconds and the first pulse module generates a first pulse every 100 microseconds. The first pulse width is then less than 100 microseconds. For example, if the first pulse width is 40 microseconds, an output of the first pulse module 702 may be high for 40 microseconds and low for 60 microseconds for each period of the first frequency.

In one embodiment, the watchdog apparatus 134 includes a first pulse average module 704 that determines a first pulse average. The first pulse average includes an average level of one or more periods of the first pulse signal. For example, where the first pulse is 40 microseconds and the first frequency is 10 kHz, the first pulse average may be 40%. The first pulse average, in one embodiment, is expressed as a voltage. For example, if the first pulse is 5 V for 40 microseconds and 0 V for 60 microseconds, the first pulse average may be 40% of 5 V, or 2 V. in other embodiments, other high and low voltages may be used and the first pulse average may differ. One of skill in the art will recognize other ways to average the first pulse signal. In one embodiment, the first pulse average module 704 determines a first pulse average for each period of the first switching frequency. In another embodiment, the first pulse average module 704 averages two or more first pulse signals. Averaging more than one first pulse signals may reduce variations due to transients and noise.

In another embodiment, the watchdog apparatus 134 includes a first pulse window module 706 that determines if the first pulse average is between a first pulse maximum threshold and a first pulse minimum threshold. For example, where the first pulse average is 40%, the first pulse maximum threshold may be 50% and the first pulse minimum threshold may be 30%. Where the first pulse average is 2 V, the first pulse maximum threshold may be 2.5 V and the first pulse minimum threshold may be 1.5V. Where the first pulse average is determined by the first pulse average module 704 to be below 1.5V or above 2.5 V, the first pulse window module 706 may determine that the first pulse average is outside of an allowable window of 1.5V to 2.5 V. The alert module 606 then sends an alert signal when the first pulse window module 706 determines that the first pulse average is not between the first pulse maximum threshold and the first pulse minimum threshold.

In one embodiment, an embodiment 700 of the watchdog apparatus 134 includes a second pulse module 708, a second pulse average module 710, and a second pulse window module 712. The second pulse module 708 generates a second pulse signal of a second frequency and the second pulse signal includes a pulse of a second pulse width. The second pulse module 708 generates a second pulse signal that is independent of the first pulse signal. In other words, the second pulse signal is generated in a way to be separate from the first pulse signal, such as on a different pin of a processor than the first pulse signal. Having a second pulse signal may provide a more fault tolerant design than an embodiment where only a first pulse signal is generated. For example, if a pin on a processor where the first pulse signal is available is shorted, the second pulse signal may be available.

In one embodiment, the first pulse width is the same as the second pulse width. In another embodiment, the first pulse width and the second pulse width are different. In another embodiment, the second pulse width is a compliment or an inversion of the first pulse width. For example, if the first pulse width is 40 microseconds starting at the beginning of a period, the second pulse width may be 60 microseconds starting 40 microseconds into the period. The first frequency may be the same or different than the second frequency.

The second pulse average module 710 determines a second pulse average that includes an average level of one or more periods of the second pulse signal, and the second pulse window module 712 determines if the second pulse average is between a second pulse maximum threshold and a second pulse minimum threshold. The second pulse average module 710 and the second pulse window module 712, in one embodiment, operate similarly to the first pulse average module 704 and the second pulse window module 712 except function with the second pulse signal. In one embodiment, the second pulse average module 710 and the second pulse window module 712 operate independently from the first pulse average module 704 and the second pulse window module 712, which may increase fault tolerance.

FIG. 8 is a schematic block diagram illustrating a third embodiment 800 of a watchdog apparatus 134 in accordance with an embodiment of the present invention. In the embodiment, the watchdog apparatus 134 is in the ground monitor apparatus 102 and includes a watchdog signal module 602 with a first pulse module 702 and a signal monitoring module 604 with a first pulse average module 704 and a first pulse window module 706, an alert module 606, a second pulse module 708, a second pulse average module 710, and a second pulse window module 712, which are substantially similar to those described in relation to the embodiment 700 of the watchdog apparatus 134 in FIG. 7. The embodiment 800 of the watchdog apparatus 134 also includes, in various embodiments, a trip module 802, a trip error module 806, a microprocessor 808, and a power supply failure module 810, and the ground monitor apparatus 102 includes a fault detection circuits 804, which are described below.

In one embodiment, the watchdog apparatus 134 includes a trip module 802 that generates a trip signal. The trip module 802 generates the trip signal in response to a fault condition in the power source 104 providing power to the load 120. For example, fault detection circuits 804 in the ground monitor apparatus 102 or other overcurrent protection device may detect a fault, such as unexpected current in the grounding conductor 118, less current or different current than expected, overcurrent, or other function where the ground monitor apparatus 102 would generate a trip signal may cause the trip module 802 to generate a trip signal. In one embodiment, the trip module 802 of the apparatus 800 of FIG. 8 is the same as the trip module 214 of the system 100 of FIG. 1 and apparatuses 200, 300 of FIGS. 2 and 3. For example, the trip module 214 of the system 100 of FIG. 1 and apparatuses 200, 300 of FIGS. 2 and 3 may include the functionality of the trip module 802 of the apparatus 800 of FIG. 8 along with functionality described in relation to the system 100 of FIG. 1 and apparatuses 200, 300 of FIGS. 2 and 3.

In the embodiment, the trip signal includes an active high signal and an active low signal where the active high signal transitions from low to high when the trip module 802 generates the trip signal and the active low signal transitions from high to low when the trip module 802 generates the trip signal. The alert module 606 sends the alert signal in response to the active high signal transitioning high and/or the active low signal transitioning low. Having an active high signal and an active low signal may provide a more fault tolerant watchdog apparatus 134.

In one embodiment, the watchdog apparatus 134 includes a trip error module 806 that sends a trip error signal in response to determining that the active high signal and the active low signal are both simultaneously high or both simultaneously low for a predetermined amount of time. For example, if the active high signal is low when it should be high and the active low signal is low, this condition may signal a fault condition, such as the active high line of the trip signal is shorted. Similarly, if the active high signal remains low when it should transition high and the active low signal transitions low, then again both the active high and the active low signals are both low, which again may indicate a fault condition. Also if both trip signals are simultaneously high, this again may indicate an unwanted condition. Any of these conditions where both the active high and the active low signals are the same causes the trip error module 806 to send a trip error signal.

In one embodiment, the trip error signal includes a communication that there is a problem with the trip signal. In another embodiment, the trip error signal causes the alert module 606 to open the contact 108. In another embodiment, the trip error signal causes the microprocessor 808, the watchdog apparatus 134, or other device to reset. One of skill in the art will recognize other ways to utilize the trip error signal.

In one embodiment, the watchdog apparatus 134 includes at least a portion of a microprocessor 808 in the ground monitor apparatus 102. For example, the watchdog signal module 602 with the first pulse module 702 and the second pulse module 708 may be portions considered part of the watchdog apparatus 134 in the microprocessor 808. By having the microprocessor 808 generate the first pulse signal and/or the second pulse signal, the watchdog apparatus 134 may provide a way to monitor health of the microprocessor 808. For example, if one or both of the first pulse signal and the second pulse signal stop functioning, change pulse width, etc., this may indicate that the microprocessor 808 is not functioning. By having the signal monitoring module 604 possibly with a first pulse average module 704 and a first pulse window module 706, the second pulse average module 710, the second pulse window module 712, and the alert module 606 external to the microprocessor 808, these external modules 604, 704, 706, 710, 712, 606 may continue to function in the event of a failure of the microprocessor 808 and can send an alert signal.

In one embodiment, the watchdog apparatus 134 includes a power supply failure module 810 that determines failure of one or more power supplies providing power to the watchdog apparatus 134. The alert module 606 sends the alert signal in response to the power supply failure module 810 determining that the analog power supply and/or the digital power supply has failed. Typically the ground monitor apparatus 102, the watchdog apparatus 134, and/or the power source 104 includes one or more power supplies that provide power to circuits, to the microprocessor 808, and to other equipment known in the art. The one or more power supplies may be analog, digital, or other type of power supply that provides power. Typically when a power supply fails, equipment receiving power from the power supply stops functioning or malfunctions. The power supply failure module 810 monitors the power supplies for failure and sends the alert signal which opens the contact 108 when the power supply failure module 810 determines that a failure condition exists in a power supply.

FIG. 9 is a schematic block diagram illustrating one embodiment of a method 900 for a watchdog function in accordance with an embodiment of the present invention. The method 900 begins and generates 902 a watchdog signal. The watchdog signal is generated at a specific interval and includes an identifiable characteristic. The watchdog signal is a part of a device being monitored for functionality, such as a microprocessor 808 or other device. The method 900 determines 904 if the watchdog signal includes the identifiable characteristic. If the method 900 determines 904 that the watchdog signal includes the identifiable characteristic, the method 900 returns and generates 902 the watchdog signal. If the method 900 determines 904 that the watchdog signal does not include the identifiable characteristic, the method 900 sends 906 an alert signal, and the method 900 ends.

FIG. 10 is a schematic block diagram illustrating another embodiment of a method 1000 for a watchdog function in accordance with an embodiment of the present invention. The method 1000 begins and generates 1002 a first pulse signal of a first frequency. The first pulse signal includes a pulse of a first pulse width. The method 1000 determines 1004 a first pulse average. The first pulse average includes an average level of one or more periods of the first pulse signal. The method 1000 determines 1006 if the first pulse average is between a first pulse maximum threshold and a first pulse minimum threshold. If the method 1000 determines 1006 that the first pulse average is between a first pulse maximum threshold and a first pulse minimum threshold, the method 1000 returns and generates 1002 the first pulse signal. If method 1000 determines 1006 that the first pulse average is not between a first pulse maximum threshold and a first pulse minimum threshold, the method 1000 sends 1008 an alert signal, and the method 1000 ends.

In addition, the method 1000 generates 1010 a second pulse signal of a second frequency. The second pulse signal includes a pulse of a second pulse width. The first pulse signal and the second pulse signal may be different or the same. The method 1000 determines 1012 a second pulse average where the second pulse average includes an average level of one or more periods of the second pulse signal. The method 1000 determines 1014 if the second pulse average is between a second pulse maximum threshold and a second pulse minimum threshold. If the method 1000 determines 1014 that the second pulse average is between a second pulse maximum threshold and a second pulse minimum threshold, the method 1000 returns and generates 1010 the second pulse signal. If method 1000 determines 1014 that the second pulse average is not between a second pulse maximum threshold and a second pulse minimum threshold, the method 1000 sends 1008 an alert signal, and the method 1000 ends.

FIG. 11 is a schematic block diagram illustrating a third embodiment of a method 1100 for a watchdog function in accordance with an embodiment of the present invention. The method 1100 begins and generates 1102 a first pulse signal of a first frequency. The first pulse signal includes a pulse of a first pulse width. The method 1100 determines 1104 a first pulse average. The first pulse average includes an average level of one or more periods of the first pulse signal. The method 1100 determines 1106 if the first pulse average is between a first pulse maximum threshold and a first pulse minimum threshold. If the method 1100 determines 1106 that the first pulse average is between a first pulse maximum threshold and a first pulse minimum threshold, the method 1100 returns to the beginning. If method 1100 determines 1106 that the first pulse average is not between a first pulse maximum threshold and a first pulse minimum threshold, the method 1100 sends 1108 an alert signal, and the method 1100 ends.

In addition, the method 1100 generates 1110 a second pulse signal of a second frequency. The second pulse signal includes a pulse of a second pulse width. The first pulse signal and the second pulse signal may be different or the same. The method 1100 determines 1112 a second pulse average where the second pulse average includes an average level of one or more periods of the second pulse signal. The method 1100 determines 1114 if the second pulse average is between a second pulse maximum threshold and a second pulse minimum threshold. If the method 1100 determines 1114 that the second pulse average is between a second pulse maximum threshold and a second pulse minimum threshold, the method 1100 returns to the beginning. If method 1100 determines 1114 that the second pulse average is not between a second pulse maximum threshold and a second pulse minimum threshold, the method 1100 sends 1108 an alert signal, and the method 1100 ends.

The method 1100 also determines 1116 if a fault condition exists in the power source 104, the ground monitor apparatus 102, etc. If the method 1100 determines 1116 that a fault condition does not exist, the method 1100 returns to the beginning. If the method 1100 determines 1116 that a fault condition exists, the method 1100 generates 1118 a trip signal and the method 1100 sends 1108 an alert signal. The method 1100 also determines 1120 if the trip signals, one being active high and another being active low, are the same. If the method 1100 determines 1120 that the trip signals are not the same, the method 1100 returns to the beginning. If the method 1100 determines 1120 that the trip signals are the same, the method 1100 sends 1108 an alert signal. The method 1100 determines 1122 if a power supply has failed. If the method 1100 determines 1122 that a power supply has not failed, the method 1100 returns to the beginning and generates 1102, 1104 the first and second pulse signals, determines 1116 if a fault condition exists, determines 1120 if the trip signals are the same, and determines 1122 if a power supply has failed. If the method 1100 determines 1122 that a power supply has failed, the method 1100 sends 1108 an alert signal, and the method 1100 ends.

FIG. 12 is a schematic block diagram illustrating one embodiment of a method 1200 for ground monitor current sensing with frequency hopping in accordance with an embodiment of the present invention. In one embodiment, the method 1200 is associated with the apparatus 201 of FIG. 2B. In another embodiment, the method 1200 is implemented with components different than shown in FIG. 2B. The method 1200 begins and injects 1202 a signal in a pilot conductor 122. In one embodiment, the signal includes a DC component and one or more AC components, for example when a zener PWD 124 is used. In another embodiment, the method 1200 injects 1202 one or more AC components without a DC component, for example, when a PWD 124 is not used or when the alternate PWD 136 is a diode oriented opposite the zener diode PWD 124 depicted in FIG. 1 where a DC component can be derived from a signal through the PWD 136 where the PWD 136 affects the current. Each AC component has a frequency different from other AC components. In one embodiment, the signal generation module 202 injects 1202 the signal in the pilot conductor 122.

The method 1200 determines 1204 if a time period has elapsed that is associated with a change in the fundamental frequency of each AC component. If the method 1200 determines 1204 that the time period has elapsed, the method 1200 changes 1206 the frequency of one or more of the AC components and the method 1200 returns and injects 1202 the new frequency or frequencies in the pilot conductor 122. In another embodiment, the method 1200 changes the phase of the one or more AC components in response to determining 1204 that the time period has elapsed. The frequency change may be simultaneous between AC components or may be staggered. If the method 1200 determines 1204 that the time period has not elapsed, the method 1200 returns and continues to determine 1204 if the time period has elapsed. In one embodiment, the frequency change module 216 determines 1204 if the time period has elapsed and changes 1206 the frequency of each AC component.

The method 1200 monitors 1208 current in the pilot conductor 122, the ground return 130, or both. The current monitor module 204, in one embodiment, monitors 1208 the current. The method 1200 determines 1210 an AC current corresponding to each frequency of the one or more AC components present in the monitored current and determines 1212 if one or more of the AC currents is below an AC threshold. If the method 1200 determines 1212 that one or more of the AC currents is not below an AC threshold, the method 1200 returns and continues to inject 1202 a signal in the pilot conductor 122. If the method 1200 determines 1212 that one or more of the AC currents is below an AC threshold, the method 1200 opens 1214 the contact 108, and the method 1200 ends. The AC detection module 208 and the AC threshold module 212 may be used in determining 1210 the AC components and if the AC components are below thresholds.

FIG. 13 is a schematic block diagram illustrating another embodiment of a method 1300 for ground monitor current sensing with frequency hopping in accordance with an embodiment of the present invention. In one embodiment, the method 1300 is associated with at least a portion of the apparatus 300 of FIG. 3. In another embodiment, the method 1300 is implemented with components different than shown in FIG. 3. The method 1300 begins and injects 1302 a signal in a pilot conductor 122. In one embodiment, the signal includes a DC component and one or more AC components, for example when a PWD 124 is used. In another embodiment, the method 1300 injects 1202 one or more AC components without a DC component, for example, when a PWD 124 is not used or when the PWD 136 is a diode oriented opposite the zener diode PWD 124 depicted in FIG. 1 where a DC component can be derived from a rectified signal through the PWD 136. In one embodiment, each AC component has a frequency different from other AC components. In one embodiment, the signal generation module 202 injects 1302 the signal in the pilot conductor 122.

The method 1300 determines 1304 if a time period has elapsed that is associated with a change in the fundamental frequency of each AC component. If the method 1300 determines 1304 that the time period has elapsed, the method 1300 changes 1306 the frequency of one or more of the AC components and the method 1300 returns and injects 1302 the new frequency or frequencies in the pilot conductor 122. In another embodiment, the method 1300 changes the phase of the one or more AC components in response to determining 1304 that the time period has elapsed. The frequency change may be simultaneous between AC components or may be staggered. If the method 1300 determines 1304 that the time period has not elapsed, the method 1300 returns and continues to determine 1304 if the time period has elapsed. In one embodiment, the frequency change module 216 determines 1304 if the time period has elapsed and changes 1306 the frequency of each AC component.

The method 1300 monitors 1308 current in the pilot conductor 122, the ground return 130, or both. The current monitor module 204, in one embodiment, monitors 1208 the current. The method 1300 determines 1310 a DC current present in the monitored current and determines 1312 if the DC current is below a DC current minimum threshold. If the method 1300 determines 1312 that the DC current is below a DC current minimum threshold, the method 1300 opens 1314 the contact 108, and the method 1300 ends. The DC minimum threshold module 210, in one embodiment, determines 1312 if the DC current is below the DC current minimum threshold. If the method 1300 determines 1312 that the DC current is not below a DC current minimum threshold, the method 1300 determines 1316 if the DC current is above a DC current maximum threshold. If the method 1300 determines 1316 that the DC current is not above the DC current maximum threshold, the method 1300 returns and continues to inject 1302 a signal in the pilot conductor 122. If the method 1300 determines 1316 that the DC current is above the DC current maximum threshold, the method 1300 opens 1314 the contact 108, and the method 1300 ends. The DC maximum threshold module 302 may be used to determine 1316 if the DC current is above the DC current maximum threshold.

The method 1300 synchronizes 1318 frequency changes, changes 1320 the sensed frequency for the one or more AC components, and determines 1322 an AC current corresponding to each frequency of the one or more AC components present in the monitored current and determines 1324 if one or more of the AC currents is below an AC threshold. If the method 1300 determines 1324 that one or more of the AC currents is not below an AC threshold, the method 1300 returns and continues to inject 1302 a signal in the pilot conductor 122. If the method 1300 determines 1324 that one or more of the AC currents is below an AC threshold, the method 1300 opens 1314 the contact 108, and the method 1300 ends. In various embodiments, the sync module 316 synchronizes 1318 frequency changes, the frequency update module 314 changes 1320 the sensed frequency of each AC component, the AC detection module 208 determines 1322 the AC components, and the AC threshold module 212 determines 1324 if the AC current is below an AC current threshold.

The method 1300 determines 1326 a current in the grounding conductor 118 and determines 1328 if the current in the grounding conductor 118 is below a return current threshold. If the method 1300 determines 1326 that the current in the grounding conductor 118 is not below the return current threshold, the method 1300 returns and continues to inject 1302 a signal in the pilot conductor 122. If the method 1300 determines 1328 that the current in the grounding conductor 118 is below the return current threshold, the method 1300 opens 1314 the contact 108. The return current module 304 and the return current threshold module 306 may be used to determine 1326 current in the grounding conductor 118 and to determine 1328 if the current in the grounding conductor 118 is below a return current threshold.

The method 1300 determines 1330 a ground fault current and determines 1332 if the ground current is above a ground fault threshold. The method 1300 may determine 1330 a ground fault current in a connection between a ground of the power source 104 and a chassis ground of the power source 104 or by sensing a differential between phase currents. If the method 1300 determines 1332 that the ground current is not above a ground fault threshold, the method 1300 returns and continues to inject 1302 a signal in the pilot conductor 122. If the method 1300 determines 1332 that the ground current is above a ground fault threshold, the method 1300 opens 1314 the contact 108, and the method 1300 ends. The ground fault current module 308 and the ground fault threshold module 310 may be used to determine 1330 a ground fault current and to determine 1332 if the ground current is above a ground fault threshold.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

What is claimed is:
 1. An apparatus comprising: a signal generation module that injects a signal in a pilot conductor, the signal comprising one or more alternating current (“AC”) components, each AC component of the one or more AC components comprising a frequency different from other AC components of the one or more AC components, wherein current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, the grounding conductor connected to a ground return; a current monitor module that monitors current in at least one of the pilot conductor and the ground return, wherein the current is affected by a rectification circuit positioned between the pilot conductor and the grounding conductor; a DC detection module that determines a direct current (“DC”) current present in the current monitored by the current monitor module; a DC minimum threshold module that determines if the DC current is below a DC current minimum threshold; an AC detection module that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module; an AC threshold module that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold; and a trip module that opens a contact in response to one or more of the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.
 2. The apparatus of claim 1, wherein the signal injected by the signal generation module is substantially free of a DC component.
 3. The apparatus of claim 1, wherein a carrier AC component of the one or more AC components has an amplitude that is greater than the amplitude of each of the other AC components of the one or more AC components.
 4. The apparatus of claim 3, wherein the carrier AC component of the one or more AC components has a fundamental frequency that is lower than the fundamental frequency of each of the other AC components of the one or more AC components.
 5. The apparatus of claim 3, wherein the AC detection module determines the AC current corresponding to each frequency of the one or more AC components during a positive half cycle of the carrier AC component when the rectification circuit allows current.
 6. The apparatus of claim 5, wherein the AC detection module deactivates determining a current during a negative half cycle of the carrier AC component when the rectification circuit blocks current.
 7. The apparatus of claim 5, wherein the AC detection module concatenates signals sensed during the positive half cycle of the carrier AC component.
 8. The apparatus of claim 7, wherein the AC detection module concatenates signals sensed during the positive half cycle of the carrier AC component by matching zero crossings of each data segment.
 9. The apparatus of claim 1, wherein the rectification circuit comprises one or more diodes.
 10. The apparatus of claim 9, wherein the rectification circuit is connected between the pilot conductor and the grounding conductor and each diode of the one or more diodes is oriented with the anode of each of the one or more diodes connected through the pilot conductor and the cathode of each of the one or more diodes connected through the grounding conductor.
 11. The apparatus of claim 1, wherein the DC detection module filters the affected current monitored by the current monitor module to detect the DC component.
 12. The apparatus of claim 1, further comprising: a frequency change module that periodically changes an AC component characteristic of each of the one or more AC components; a frequency update module that changes a frequency sensed for each of the one or more AC components to match the fundamental frequencies of the AC components in response to the frequency change module changing the AC component characteristic by changing the fundamental frequency of each of the one or more AC components; and a sync module that synchronizes changes in sensing by the frequency update module with changes to the fundamental frequencies of the AC components by the frequency change module.
 13. The apparatus of claim 1, wherein the frequency change module changes the AC component characteristic of each of the one or more AC components by changing one or more of: a frequency of each of the one or more AC components; and a phase of each of the one or more AC components.
 14. The apparatus of claim 13, wherein the frequency change module changes the AC component characteristic of each of the one or more AC components by one or more of: ramping from a current frequency to a new frequency over a period of time; fading out a first frequency generator with a current frequency and fading in a second frequency generator with a new frequency; ramping from a current phase to a new phase over a period of time; and fading out a first frequency generator with a current phase and fading in a second frequency generator with a new phase.
 15. The apparatus of claim 1, further comprising a DC maximum threshold module that determines if the DC current is above a DC current maximum threshold and wherein the trip module further opens the contact in response to the DC maximum threshold module determining that the DC current is above the DC maximum current threshold.
 16. The apparatus of claim 1, further comprising: a return current module that determines a current in the grounding conductor; and a return current threshold module that determines if the current in the grounding conductor is below a return current threshold, wherein the trip module further opens the contact in response to the return current threshold module determining that the current in the grounding conductor is below the return current threshold for a period longer than a return current time threshold.
 17. A system comprising: a power source; a ground monitor in the power source, the ground monitor comprising a signal generation module that injects a signal in a pilot conductor, the signal comprising one or more alternating current (“AC”) components, each AC component of the one or more AC components comprising a frequency different from other AC components of the one or more AC components, wherein current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, the grounding conductor connected to a ground return; a current monitor module that monitors current in at least one of the pilot conductor and the ground return, wherein the current is affected by a rectification circuit positioned between the pilot conductor and the grounding conductor; a DC detection module that determines a direct current (“DC”) current present in the current monitored by the current monitor module; a DC minimum threshold module that determines if the DC current is below a DC current minimum threshold; an AC detection module that determines an AC current corresponding to each frequency of the one or more AC components present in the current monitored by the current monitor module; an AC threshold module that determines if one or more of the AC currents determined by the AC detection module is below an AC threshold; and a trip module that opens a contact in response to one or more of the DC threshold module determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and the AC threshold module determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.
 18. A method comprising: injecting a signal in a pilot conductor, the signal comprising one or more alternating current (“AC”) components, each AC component of the one or more AC components comprising a frequency different from other AC components of the one or more AC components, wherein current in the pilot conductor is injected in a grounding conductor of a set of power cables connecting a power source to a load, the grounding conductor connected to a ground return; monitoring current in at least one of the pilot conductor and the ground return, wherein the current is affected by a rectification circuit positioned between the pilot conductor and the grounding conductor; determining a direct current (“DC”) current present in the monitored current; determining if the DC current is below a DC current minimum threshold; determining an AC current corresponding to each frequency of the one or more AC components present in the monitored current; determining if one or more of the AC currents determined by the AC detection module is below an AC threshold; and opening a contact in response to one or more of determining that the DC current is below the DC current minimum threshold longer than a DC current minimum time threshold and determining that at least one of the AC currents is below an AC threshold for longer than an AC current time threshold, the contact disconnecting the power source from the set of power cables.
 19. The method of claim 18, wherein a carrier AC component of the one or more AC components has a fundamental frequency that is lower than the fundamental frequency of each of the other AC components of the one or more AC components and the carrier AC component of the one or more AC components has an amplitude that is greater than the amplitude of each of the other AC components of the one or more AC components.
 20. The method of claim 19, wherein determining the AC current of the one or more AC components comprises determining the AC current corresponding to each frequency of the one or more AC components during a positive half cycle of the carrier AC component when the rectification circuit allows current and deactivating determining a current during a negative half cycle of the carrier AC component when the rectification circuit blocks current. 